Roger Weller, geology instructor firstname.lastname@example.org
on Mars.... Where is it Hiding?
There are no lakes on Mars. Thousands of detailed photos of Mars have revealed no lakes. This was expected. The atmosphere on Mars is roughly only 1 percent of Earth's atmospheric pressure. Under this extremely low pressure the boiling temperature of water drops almost down to its freezing point. Water exposed to the atmosphere would quickly boil away. Ice, another form of water when exposed to the low atmospheric pressure would sublimate; this means that ice would simply evaporate as water vapor.
So, where could water be hiding?
On Earth, there is water in
the ground, located between mineral grains. We refer to this water as
"groundwater". In Cochise County, Arizona this is our primary source of
drinking water. The big question is, "How do we get this water out of the
There are two concepts that we need to understand before we go after the groundwater: POROSITY and PERMEABILITY.
POROSITY is the measure of
open space between the mineral and rock grains in the ground. The more open
space, the more room for the open space to be filled with water.
The greatest amount of open space between grains exists when the grains are all about the same size and somewhat rounded.
Sandstone, a rock consisting of sand grains cemented together, has between 8% to 35% open space, depending upon the amount of natural cement holding the grains together. Siltstone, a rock made of silt grains, which are finer than sand, has porosities ranging from 14% to 43%.
Both sandstone and siltstone are common on Earth because erosion breaks down the rocks and flowing rivers or sandstorms can carry these grains away and deposit them in large quantities.
On Mars there are no flowing rivers and there not have been away for billions of years. The primary agent of erosion on Mars today are dust storms in which only silt-sized particles can be carried by wind in the thin atmosphere. The material covering the ground on Mars is likely to be a layer of silt particles. These silt particles are also lightly packed together because of the lower gravity on Mars which is about one third of Earth gravity. All of this discussion means that there might be a high porosity in Martian soils.
Take a clear plastic
container with a wide mouth. Fill the container half full of dry sand. Make a
water solution that is strongly colored with food dye. Slowly pour the solution
very slowly into the container of sand. Measure how much solution the sand can
store. You will be able to see the colored solution filling the spaces in the
Repeat the experiment using
small pebbles instead of the sand.
PERMEABILTY is a measure of how easy it is for a fluid to flow through a material. In the experiment that you just performed, did you notice how fast the water soaked into the gravel in comparison to the sand? The open spaces in the gravel are much larger than those in the sand, so there is less friction slowing the water down. The loose gravel is much more permeable than the sand. On Earth, much of our groundwater comes from buried gravels and sand, because these materials have high porosity and permeability.
On Mars we expect to find silt instead of nicely sorted sand and gravel. Because silt grains are so much smaller than sand, friction against the flow of water will be much greater. In other words, what we expect to find on Mars is that the silt will have a good porosity for storing water, but a low permeability for allowing the water to flow.
Assuming that the silt on Mars is porous and permeable, there is still the problem with the low atmospheric pressure on Mars. Water would still boil way or ice, frozen between the mineral grains, would still sublimate. In order to store water in the ground and keep it from disappearing into the atmosphere, there needs to be a capping layer of some sort on top of wet silt that is IMPERMEABLE (will not allow water to flow through). Currently, we do not know if such a layer might exist.
One process that might be able for creating a cap to stop the evaporation water would be a layer of minerals were deposited from solution as mineral-rich water evaporated. This layer would be something that Mars explorers would be looking for. This layer would be something that Mars explorers would be looking for.